1. Field of the Invention
The present invention relates generally to a wireless communication system, and in particular, to a method and apparatus for configuring a channel node tree to allocate resources in an Orthogonal Frequency Division Multiple Access (OFDMA) wireless communication system.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) has recently been actively studied and utilized for wireless communication systems. OFDM is a special type of multi-carrier modulation, in which a serial symbol sequence is converted to parallel symbol sequences and modulated to a plurality of orthogonal subcarriers, that is, subcarrier channels, prior to transmission.
FIG. 1 illustrates time-frequency resources in a conventional OFDM wireless communication system.
The OFDM system typically transmits one modulation symbol such as a Quadrature Phase Shift Keying (QPSK) or 16-ary Quadrature Amplitude Modulation (16QAM) symbol on one subcarrier. Thus, subcarriers are basic resources.
Referring to FIG. 1, the horizontal axis represents time and the vertical axis represents frequency. Reference numeral 101 denotes a subcarrier and reference numeral 102 denotes an OFDM symbol.
In general, one OFDM symbol includes a plurality of subcarriers. It is assumed herein that the subcarriers illustrated in FIG. 1 are those that carry actual data, without guard subcarriers. As denoted by reference numeral 103, a plurality of OFDM symbols form a basic packet data transmission unit called a “frame”. As illustrated in FIG. 1, one frame includes a plurality of OFDM symbols, each having a plurality of subcarriers. One rectangle represents a subcarrier in an OFDM symbol, referred to as “time-frequency resources”.
Typically, one frame includes a plurality of physical channels. For example, the time-frequency resources of one frame are used for a Paging CHannel (PCH), a Common Control CHannel (CCCH) for carrying system information, a Packet Data CHannel (PDCH) for transmitting user data and a Packet Data Control CHannel (PDCCH) for transmitting control information necessary for demodulation of the PDCH. There may also exist other physical channels that serve other purposes.
Since the OFDM wireless communication system has two-dimensional resources, i.e. time-frequency resources and uses a plurality of physical channels with different resource requirements, subcarriers should be efficiently allocated to each physical channel and indicated between a transmitter and a receiver. For this purpose, the OFDM wireless communication system defines a basic resource allocation unit, allocates resources to each physical channel in basic resource allocation units, and indicates the positions of the resources between the transmitter and the receiver by an indicator for the resource allocation units. Hereinbelow, the basic resource allocation unit is referred to as a “resource channel”.
The two resource channels in an OFDMA system are a Distributed Resource CHannel (DRCH) and a Block Resource CHannel (BRCH).
The DRCH includes subcarriers distributed in time and frequency, and the BRCH includes successive subcarriers.
The DRCH is used to maximize diversity gain, whereas the BRCH is used to maximize channel selective resource scheduling gain.
FIG. 2 illustrates a conventional configuration of the DRCH.
Referring to FIG. 2, DRCH (D, k) represents a kth resource channel among D resource channels formed from total subcarriers available to one OFDM symbol. As illustrated in FIG. 2, a DRCH has scattered resources, which suits diversity transmission. In an OFDMA system, different resource channels are allocated to users by DRCH (D, k) indicators.
FIG. 3 illustrates a conventional configuration of the BRCH.
Referring to FIG. 3, one BRCH includes 16 successive subcarriers and 8 successive OFDM symbols, thus a total of 128 time-frequency resources. This BRCH is a basic resource allocation unit.
Like the DRCH, BRCH (B, k) represents a kth resource channel among B resource channels formed from total subcarriers available to one OFDM symbol.
FIG. 4 illustrates a conventional resource allocation for multiplexing a DRCH with a BRCH in one frame.
Referring to FIG. 4, DRCHs are multiplexed with BRCHs. When a DRCH overlaps with a BRCH, the DRCH punctures a portion of the BRCH. This multiplexing method is useful in achieving both diversity gain and channel selective scheduling gain from one frame.
Meanwhile, DRCHs and BRCHs are allocated by configuring a channel node tree and allocating one node of the channel node tree.
FIG. 5 illustrates a conventional channel node tree.
Referring to FIG. 5, the lowest nodes are called base nodes 510 and every pair of base nodes are connected to a second-level node 520. As illustrated in FIG. 5, the base nodes 510 and the second-level nodes 520 are at different node levels. In this context, there are six node levels for the channel node tree.
The channel node tree has 30 base nodes 510, which indicates that D is 30 for DRCH (D, k) and B is 30 for BRCH (B, k). That is, DRCH (30, 0) to DRCH (30, 29) are mapped to the base nodes 510 and DRCH resources are allocated by allocating one node of the channel node tree. BRCH resources are allocated in the same manner.
If a DRCH is multiplexed with a BRCH as illustrated in FIG. 4, a DRCH channel node tree and a BRCH channel node tree are configured separately. When resources are allocated, one node of each channel node tree is allocated along with a node ID identifying a DRCH or a BRCH. A user to which BRCH resources are allocated can use the remaining BRCH resources except BRCH resources overlapped with DRCH resources in a frame.
Because two channel node trees are configured for DRCH and BRCH allocations, a problem occurs in that more signaling bits are used to indicate the DRCH and the BRCH allocations and more resources are required to transmit uplink ACKnowledgement/Negative ACKnowledgement (ACK/NACK) for downlink Hybrid Automatic Repeat reQuest (HARQ).